1. Field of the Invention
The present invention relates to a correlation time-difference detector and more particularly to a correlation time-difference detector for measuring a relative time difference between two analog input signals having correlation with each other.
2. Description of the Prior Art
FIGS. 4 and 5 exemplify structure of a prior art correlation time-difference detector of the mentioned type. The prior art exemplified there is of the type in which the input analog signals are converted into binary signals and an exclusive NOR (EX-NOR) circuit is used for multiplication and summation thereof. Referring now to FIG. 4, there is exemplified structure of a correlation function computing element 100 being used in the correlation time-interval detector. In FIG. 4, 1 denotes a clock oscillator, 2 denotes a counter, 3 and 4 denote comparators, 5 denotes a shift register, 6 denotes a multiplexer, 7 denotes a latch, 8 denotes an exclusive NOR gate, and 9 denotes a memory.
Operation of the same will be described in the following. The counter 2, in response to the output signal from the clock oscillator 1, outputs address signals for the multiplexer 6 and memory 9, a shift signal for the shift register 5, a latch signal for the latch 7, and read/write signals (R/W) for the memory 9. The counter 2 is a preset counter in which the number of bits of the shift register 5, corresponding to its number of stages, is established as its preset number, and a carry signal (Co) to be generated thereby each time the full count is reached therein is used as the above mentioned shift signal and latch signal. The two analog signals as the objects of the measurement are input to comparators 3 and 4, respectively, and, depending on their being positive or negative, signals converted into a binary form, namely, such as to take the value "1" when positive, and the value "0" when negative, are output therefrom. The aforesaid binary output signals from the comparators 3 and 4 are sent to the shift register 5 and latch 7 in response to the carry signal (Co) to be generated by the counter 2 each time the full count is reached. In the shift register 5 is stored the data in the past indicating the period of time corresponding to the product of its number of bits and the period of the shift signal equivalent to the carry signal. Meanwhile, during one period of the carry signal, the counter 2 increases the address signal for the multiplexer 6 corresponding to the bits of the shift register 5 successively from its front to rear, whereby the data in the shift register 5 are read out in succession and supplied to the exclusive NOR 8 by way of the multiplexer 6. The exclusive NOR 8 also receives an output signal from the latch 7 and compares the same with the output signal from the multiplexer 6 and outputs "1" if these agree with each other and outputs "0" otherwise. Supposing now, out of the two analog signals, the one for the comparator 3 having a time difference preceding the other, probability for two input signals to agree with each other will be higher when the multiplexer 6 is reading the data in the shift register 5 in the vicinity of the address corresponding to the preceded time interval, hence the probability of the output of the exclusive NOR 8 to become "1" will then be higher. On the other hand, the correlation between the signals corresponding to other time intervals than that will be lower, and so the probability for the output to become "1" will be lower. That is, average value of the output of the exclusive NOR 8 becomes high when the read out data correspond to the time difference between the two signals. The variations of this output approximately represent the cross-correlation function between the two analog signals. Therefore, by knowing the time interval, namely, the corresponding address in the shift register, which makes the correlation function maximum, the time interval between the inputs of the two analog signals can be determined. But, only one time of scanning by the multiplexer 6 does not provide an accurate correlation function. Therefore, in order to improve the accuracy, it is the practice to repeat the scanning and accumulate the computed results in the memory 9 and to take an average therefrom.
Then, referring to FIG. 5, there is schematically indicated the structure of a correlation time-difference detector including the correlation function computing element as shown in FIG. 4. In FIG. 5, 100 denotes the correlation function computing element, 200 denotes a specific computing element such as a microprocessor, and 300 denotes a data bus. In this arrangement, the chief function of the computing element 200 is to determine the position of the peak of the correlation function obtained by the correlation function computing element 100. The computing element 200 cyclically drives the correlation function computing element 100 to cause the computed results of the correlation function to be further accumulated in the memory 9 (FIG. 4) for improvement of the accuracy. After sufficiently high accuracy is thus obtained, the peak position of the correlation function is computed. The peak position is computed by means of fitting by parabolic function, or normal distribution function, or center-of-gravity calculation. The product of the value of the peak position of the correlation function expressed by the address in the shift register 5 (FIG. 4) thus obtained and the period of the carry signal gives the time difference between the two analog signals under measurement.
The prior art correlation time-interval detector as described above had a problem that, in order to compute the peak position of the correlation function it, had to have a computing element such as a microprocessor provided at the succeeding stage to the correlation function computing element. It was also a problem for the prior art correlation time-difference detector that it could provide the value of measurement only intermittently.